Is there a way to rotate view by 180 or 90 degrees with 1 command?
Method 1
You can go into the reverse of an orthographic view (i.e. the view is rotated by 180 degrees) by adding Ctrl to the numpad number. For example Numpad 1 goes into front view but Ctrl+Numpad 1 goes into back view. This works for Numpad 1, Numpad 3 and Numpad 7.
So you can either press Numpad 1 to go into front view and then press Ctrl+Numpad 1 to reverse that view or you can press Ctrl+Numpad 1 to go directly into that view.
Method 2
It is currently taking you 12 presses to rotate the view using the number pad keys because the 'Rotation Angle' is set to 15 (degrees). Luckily, there is a user preference option to set this to a custom angle (the maximum value it can be set to is 90 degrees).
In 2.8 you can go to Edit&gt; Preferences&gt; Navigation&gt; Orbit &amp; Pan and change the 'Rotation Angle' from there: